Digital signal compression is widely used in many multimedia applications and devices. Digital signal compression using a coder/decoder (codec) allows streaming media, such as audio or video signals to be transmitted over the Internet or stored on compact discs. A number of different standards of digital video compression have emerged, including H.261, H.263; DV; MPEG-1, MPEG-2, MPEG-4, VC1; and AVC (H.264). These standards, as well as other video compression technologies, seek to efficiently represent a video frame picture by eliminating the spatial and temporal redundancies in the picture and among successive pictures. Through the use of such compression standards, video contents can be carried in highly compressed video bit streams, and thus efficiently stored in disks or transmitted over networks.
A very efficient and commonly used tool for redundancy removal is known as motion compensation. In the motion compensation process, the codec tries to find a best matched reference pixel block in an already coded video image to represent a currently processing pixel block. Then, instead of coding pixels in the current block, only relative spatial and temporal offsets between the currently processing block and the reference block and motion compensation residue are coded into the output video stream. As the result, the number of bits used to encode this pixel block may be much smaller than if all the pixels were to be encoded directly.
Another commonly-used coding tool is known as intra prediction. Intra prediction uses neighboring pixels or pixel blocks to predict a currently processing pixel block. But, on average, the coding efficiency of intra prediction is much lower than best match based motion compensation used in inter prediction.
As defined in existing motion compensation enabled video coding standards, H.261, H.263, MPEG-1, MPEG-2, MPEG-4, VC1 and AVC (H.264), motion compensation only happens between two different pictures. As a result, the currently processing picture must be motion predicted by reference to another picture that has already been processed. In most cases, this constraint would not cause any problem and the coder always can find a best prediction from a previously coded picture for the current picture. However, a previously coded picture may not always be available or may not be the best reference picture to use for motion compensation. For example, if the current picture is intra coded, there is no other picture available that can be used as a reference picture.
The definition of intra picture is that all predictions have to be done within the same picture. With current available video codec standards, only intra prediction can be used in an intra picture.
It is within this context that embodiments of the present invention arise.